1. Field of the Invention
The present invention relates to an optical pick-up device for use in an optical disk apparatus such as a magneto-optical disk recording and reproducing apparatus.
2. Description of Related Art
An optical pick-up device for use in a magneto-optical disk apparatus has been conventionally designed as shown in FIG. 1. In FIG. 1, an optical pick-up device 1 includes a laser beam source 2 such as a semiconductor laser element or the like and a prism 3 having a reflecting surface for reflecting a laser beam emitted from the laser beam source 2 in such a direction that the optical path of the laser beam is bent vertically from the surface of the drawing by 90 towards the viewer. The optical pick-up device 1 further includes an objective lens (not shown) which is provided just before the prism 3, a grating 4, a beam splitter 5 and a collimator lens 6 which are disposed, in the order given, between the laser beam source 2 and the prism 3, a composite lens 7 and a cylindrical lens 8 which are disposed at the lower side of the beam splitter 5 in FIG. 1, and a photodetector 9 disposed at the lower side of the cylindrical lens 8.
In the pick-up device 1 thus constructed, the laser beam emitted from the laser beam source 2 passes through the grating 4, the beam splitter 5 and the collimator 6 and is reflected from the prism 3. The laser beam reflected from the prism 3 passes through the objective lens (not shown) and is focused on the surface of a magneto-optical disk (not shown).
The laser beam is reflected from the surface of the magneto-optical disk, passes through the objective lens again, is reflected from the prism 3, and then travels along a return optical path. This reflected laser beam passes through the collimator lens 6 and then enters the beam splitter 5. The laser beam which is reflected from the magneto-optical disk and then enters the beam splitter 5 is reflected from a reflecting surface 5a of the beam splitter 5 and into the photodetector 9 via the composite lens 7 and the cylindrical lens 8. Upon receiving this reflected laser beam, the photodetector 9 detects a focusing control signal, a tracking control signal and a reproduction signal.
In the pick-up device 1, the focusing control signal, the tracking control signal and the reproduction signal are all detected by the photodetector 9. Among these signals, the focusing control signal in particular requires accurate positioning between the photodetector 9 and the reflected laser beam. Accordingly, the photodetector 9 must be accurately positioned, and this requirement makes fabrication of the pick-up device more troublesome.
Further, environmental variations in characteristics and performance such as variations in temperature, time lapse variations, etc. cause positional deviations of the relative positional relationship between the optical axis of the photodetector 9 and the center of the laser beam. Positional deviation between the optical axis of the photodetector 9 and the center of the reflected laser beam obstructs the achievement of accurate focusing control.
Still further, the reflected laser beam on the light-receiving surface of the photodetector 9 has a spot size of several tens of microns, so that the photodetector 9 itself is relatively large in size. Accordingly, the degree of freedom with which the photodetector 9 is arranged in the pick-up device is lowered, with the result that the pick-up device as a whole cannot be significantly reduced in size.